1. Field of the Invention
The present invention relates to a rotary steerable system for use in drilling a deviated well.
2. Description of the Prior Art
When drilling a hole in a subsurface formation, it is often desirable to be able to vary and control the direction of drilling. This is particularly true in the case of drilling a deviated well, i.e., one whose borehole is not perpendicular to the earth.
Rotary drilling is defined as a method in which a bottom hole assembly, including the drill bit, is connected to the drill string which is rotatably driven from the drilling platform at the surface. Until recent years, fully controllable directional drilling has normally required the drill bit to be rotated by a downhole motor or a turbine. The drill bit may then, for example, be coupled to the motor by a bent unit whereby the central axis of the drill bit is inclined with respect to the axis of the motor. When variation of the direction of drilling is required, the rotation of the drill string is stopped, and the drill string oriented in the new direction. Continued rotation by the drill bit by the motor then causes the bit to drill in the new direction. Although such arrangements can, under favorable conditions, permit accurately controlled directional drilling to be achieved, rotary drilling is a preferred form of drilling. Rotary drilling allows one to drill a smoother trajectory of the well, and also reduces friction of the drill string on the low side of the hole, therefore allowing a better transmission of the weight from the surface down to the drill bit.
Accordingly, in recent years, attention has been given to the development of arrangements for achieving a rotary steerable system. Some of those systems are for connection in the bottom hole assembly of a drill string and have comprised a number of hydraulic actuators spaced apart around the periphery of the unit. Each of the actuators has a moveable thrust member or pad which is hydraulically displaceable outwardly for engagement with the formation of the borehole being drilled. The rotary steerable system also includes a selector apparatus which, when actuated, causes each of the moveable thrust members to be displaced outwardly at the same selective rotational position, which biases the drill bit laterally and thus controls the direction of drilling.
Prior art rotary steerable systems, in addition to being rather complex, have used the drilling fluid to actuate the moveable thrust members or pads. No prior art rotary steerable system to applicant's knowledge has contained a reference device which utilizes a simple mechanical device to provide a gravity reference.